Voice-over-IP audio mixer devices are used for implementing conferencing applications for IP networks. Audio signals, after being converted to IP packets are sent to these devices and the audio signals are mixed together and then output to the users such that they can hear a complete conference conversation without hearing their own voice. Such applications sometimes only mix certain users within the conference based on a criteria that they are the ones contributing to the conference.
User signal events are used to send information from one device to another. Over a voice-over-IP network, these signals may be embedded as part of the audio signal within the IP packet, such as in-band DTMF (Dual-tone Multi-frequency) or they may be sent as separate out of band packets, as RFC 2833 and RFC 4733.
There are several scenarios where it is useful to pass user signal event digits through a conference on an IP mixer device. In the first example, a customer support specialist might wish to help a new user setup a mailbox. The support specialists would need to have their DTMF digits reach the user's mailbox during the call. In the second example, during an attempt to add another member to a conference call, the operator reaches the member's mailbox. DTMF digits must reach the mailbox in order to drop the call in an expedient manner. This is far more effective than waiting or dropping the conference call and re-dialling. In the third example, while on a conference call, someone needs to confirm travel plans. He/she can dial into an IVR (Interactive Voice Response) system during the conference call to obtain confirmation. This again requires user signalling event pass-through.
Currently, voice-over-IP audio mixer devices have difficulty knowing how to handle these user signalling events for a conference. These signals are currently not passed through a conference application because they are either removed or distorted. In-band digits are normally removed by a DTMF clamping feature. Not being part of the audio stream, RFC 2833 or RFC 4733 digits also never reach the conference mixer. If the IP mixer does not clamp the DTMF signals found in the audio stream then the digits will pass through the conference mixer but are likely to become distorted in the mixing process. However, RFC 2833 and RFC 4733 signal events can never pass through as they are not part of the audio stream since they are signalled out of band.
While DTMF signalling events can be passed through a conference by not clamping these signals, this would only solve 1 of 4 possible scenarios required for passing through the events, namely the case where the sending and receiving users are both using in-band DTMF signalling events. The 3 other cases, RFC 2833/4733 to in-band DTMF, in-band DTMF to RFC 2833/4733, and RFC 2833/4733 to RFC 2833/4733 would not function. In addition, the quality of the in-band DTMF digits would be degraded by being mixed with the audio from other conference users thereby reducing the chances of proper digit detection at the receiving end.
Another issue is that conference mixers use algorithms that do not always mix all users at any moment during the conference. If an in-band event is present on one of the user's input but that user is not currently contributing audio to the conference mix then the digit will not be present on the output of the conference. Thus, a method is needed to guarantee that a digit will always be passed through to the output of the conference regardless of the signal event type or the operation of the conference mixer.